The transmission medium extending between a line circuit in a switching facility such as a PBX, key service unit or central office and a telephone set must provide a communications link that provides a minimum level of attenuation and impairment in order to assure reliable two-way communications. Among other things, the transmission equipment and protocol determine the minimum signal level threshold required to assure reliable communication over links of varying length and impairments.
Typically, when a digital telephone set is installed on a line which connects it to switching equipment (central office, PBX, key system, etc.) the installer is not provided with a means to determine how well the communications path between switch and telephone set are functioning. The telephone set either works flawlessly or doesn't work at all. Only rarely will the telephone set exhibit marginal performance. These characteristics are due to the nature of digital communications and are well understood in the industry. As long as the data are received above some minimum signal to noise threshold the receiving device works flawlessly. In linear systems with Gaussian noise, the difference in signal to noise ratio between nonoperation and flawless operation of a digital receiving device may be as small as 1/2 dB. This invention provides a means for an installer (as well as a maintenance center or even the user) to measure and determine a figure of merit regarding the performance of the link between telephone set and switching equipment at installation time, or any time thereafter. If the figure of merit is low, the installer can then investigate the reason for that low figure of merit or turn the matter over to another organization that may have greater expertise and/or more extensive test and diagnostic equipment to correct the situation.